Facilities such as warehouses, distribution facilities and parcel and document delivery depots process an ever increasing flow of packages, products and other items (hereinafter, for convenience, "objects") daily. Such facilities frequently utilize a system of automated, fixed conveyors to route and deliver simultaneously objects from storage space or processing containers to trucks, airplanes, or containers, each of which corresponds to a different address or destination. It is not unusual for regional warehouses of large mail order merchandising companies, for instance, to have fifty or more tractor trailers parked at their loading docks simultaneously receiving objects from the automated conveyor system.
Objects travelling on these facility conveyor systems are typically being tracked by a system of optical or mechanical sensors and automated equipment. Although such conveyor systems may be arranged and operated in virtually any number of ways, such equipment generally tracks each object as it progresses along an arterial conveyor which handles all or a substantial quantity of the objects. The equipment then generally shunts each object off of the arterial conveyor, when it has reached an appropriate location, onto a branch or spur conveyor which corresponds to a truck, airplane or container (hereinafter, for convenience, "container").
In many applications, the arterial and spur conveyors deliver objects at a constant velocity, and with no interruption, directly into the interior of the container. Workers must then remove the objects, turn, load them and repeat the motion until a location in the container is filled. They then must extend or retract the conveyor and perhaps swing it in order to fill a new location, while still dealing with the flow of objects. Workers unloading such conveyors are accordingly subject to the constant strain of offloading an uninterrupted flow of objects, and the accompanying stress caused by feeling that they have no control over that flow.
The environment inside such facilities is, furthermore, typically inconducive to the welfare of finely tuned conveyor equipment, and spur conveyors consequently usually feature heavy duty structure such as I beam or C beam longitudinal members which can withstand, for instance, an inadvertent fork lift collision, and roller supporting structure which can withstand the constant abuse of objects being dropped on them. Such robust structures tend, however, to be heavy and thus difficult to extend, retract or otherwise reposition without straining the workers.
Recent increased attention to ergonomics and worker welfare under other rubrics (and consequent control of health care costs) have created economic incentives for facilities to employ spur conveyors that reduce the stress and strain on workers using them. Among other measures, conveyors using lazy tong structures have been employed. These are easily expandable and retractable, and may be twisted and curved into position without adversely affecting flow of objects. Surprisingly, their mechanical flexibility lends great durability and strength for the taxing physical environment, like a willow in the wind.
Lazy tongs are structures formed of a set of parallel bars, each of which is connected in pivoting fashion at its end points and, in some cases, at its midpoint, to corresponding bars in a transversely oriented set of parallel bars so that the structure may be extended and retracted with scissor-like action. Such structures are disclosed in, for instance, U.S. Pat. No. 5,147,025 to Flippo and U.S. Pat. No. 4,852,712 to Best which disclose lazy tong conveyors, powered and unpowered, respectively, and which are incorporated herein by this reference. The Best patent discloses non-powered flexible, expandable conveyors, while the Flippo patent discloses powered conveyors that employ belting or chain to actuate rollers.
Lazy tong roller conveyors as disclosed in the Best and Flippo patents are particularly useful to move objects from one point to other points in a plant or warehouse, and in loading or unloading containers. They may be flexed and extended or retracted with minimum effort to accommodate the desired location, vehicle or container. The nature of the lazy tong structure also allows such conveyors to be retracted and stored in a compact fashion in which successive rollers are positioned nested together immediately adjacent to one another.
The Flippo patent discloses lazy tong flexible and expandable roll conveyors which use drive belting or chain interlaced over rollers on the conveyor surface to propel articles along the conveyor surface. As the conveyor is expanded or retracted, the interlaced belts retain approximately their same length and are thus able to actuate the drive rollers on the top surface and convey objects at any desired conveyor length or curve.
Lazy tong conveyors thus are gaining popularity for use as the final portion of spur conveyors; their flexibility and light weight allows them to perform optimally in transitioning objects from the end of a fixed spur conveyor to an ever changing location in a container, with minimal stress and strain on workers. Their resilient structure lends durability not found in heavier, more rigid previous structures such as I beam conveyors on casters.
Chain driven conveyors, such as the ones disclosed in the Flippo patent, operate effectively in many ways when employed to transition the flow of objects from a constant velocity facility conveyor system to workers offloading objects into a container. However, when such workers get behind in offloading or otherwise interrupt the offloading process, they have no option but to shut down the entire chain driven transition conveyor and thus cause accumulation of objects on the branch portion of the facility conveyor system. In short, such a chain driven conveyor is conceptually simply part of or an extension of the constant velocity conveyor system and it allows no flexibility in order to accommodate the intermittent nature of the manual offloading process.